1. Field of the Invention
This invention relates to an active-type organic electroluminescenct (EL) display device that includes thin-film transistors (TFTs) driving EL elements, and in particular, an active-type organic EL display device that provides an easy brightness adjustment.
2. Description of the Related Art
Since organic EL elements are self-luminous, they requires no back lights as required in liquid crystal display devices and are optimal for a reduction in thickness. Moreover, since-they have no limit in viewing angle, practical application thereof as a next-generation display device has been greatly expected.
In display devices using such organic EL elements, different luminescent materials for respective RGB three primary colors are used in corresponding light emitting layers, and are independently disposed corresponding pixels to directly emit respective RGB lights.
In such an organic EL display device, as shown in FIG. 1, RGB image signals are corrected by respective gamma correction circuits 71, 72, and 73 individually provided for the RGB signals. The gamma corrected signals are supplied to an organic EL panel 70 to display an image. Gamma correction converts the relationship where the output luminance level is proportional to the input signals to the power of gamma into the relationship where the output luminance is linearly proportional to input signals.
In FIG. 2, luminance characteristics of respective RGB light emitting layers are shown on the left side, the characteristics between the input gradation and luminance corrected by the gamma correction circuits 71, 72, and 73 are shown on the right side. Namely, in order to maintain white balance, luminance ratio of RGB is determined in an order of GBR, and gamma correction is carried out by the respective RGB gamma correction circuits 71, 72, and 73 so that RGB luminance vary linearly to allow a 64-gradation display.
Since R is driven with a luminance between Rmin and Rmax, it is sufficient to adjust a 64-gradation voltage within a range xcex94R shown by arrows for a voltage applied to the R light emitting layer. Since G is also driven with a luminance between Gmin and Gmax, it is sufficient to adjust a 64-gradation voltage within a range xcex94G shown by arrows for a voltage applied to the G light emitting layer. Similarly, since B is also driven with a luminance between Bmin and Bmax, it is sufficient to adjust a 64-gradation voltage within a range xcex94B shown by arrows for a voltage applied to the B light emitting layer.
However, in the aforementioned organic EL display device, although an adjustment in luminance of the respective light emitting layers is possible for a 64-gradation display within the ranges of RGB image signals, the total luminance of the organic EL display device cannot be controlled to reflect the surrounding environment conditions. In particular, the whole image can not be made brighter in an environment of daytime outdoor use, or the whole image can be made darker in an environment of nighttime indoor use.
In addition, in the aforementioned organic EL display device, as shown in FIG. 2, since the luminance setting ranges of the respective RGB light emitting layers, namely, the range xcex94R, the range xcex94G and the range xcex94B, are different, the output dynamic range of an IC that outputs image signals cannot cover all of the ranges xcex94R, xcex94G and xcex94B. In particular, the high range of xcex94B exceeds the output dynamic range. Accordingly, the luminance cannot be effectively controlled in response to the image signals in the B light emitting layer.
The invention provides an organic electroluminescent display device that includes a plurality of electroluminescent elements. Each of the electroluminescent elements includes an anode, a cathode and a light emitting layer disposed between the anode and the cathode. The device also includes a plurality of thin film transistors. Each of the thin film transistors drives one of the electroluminescent elements. A first power supply is connected to the thin film transistors and supplies a power supply voltage to the thin film transistors. The first power supply changes the power supply voltage so that luminance of the light emitting layers vary. A second power supply is connected to the electroluminescent elements and supplies a reference voltage to the electroluminescent elements.
The invention also provides an organic electroluminescent display device that includes, for each of red, green and blue emissions, a plurality of electroluminescent elements for corresponding light emission. Each of the electroluminescent elements includes an anode, a cathode and a light emitting layer disposed between the anode and the cathode. The device also includes a plurality of thin film transistors, each of which drives one of the electroluminescent elements. The device includes, for each of red, green and blue emissions, a power supply connected to the thin film transistors for supplying a power supply voltage. The power supply changes the power supply voltage so that luminance of the corresponding light emitting layers vary. The device also includes a reference voltage supply connected to the electroluminescent elements for supplying a reference voltage.
The invention further provides an organic electroluminescent display device that includes a plurality of electroluminescent elements. Each of the electroluminescent elements includes an anode, a cathode and a light emitting layer disposed between the anode and the cathode. The device also includes a plurality of thin film transistors and a sensor. Each of the thin film transistors drives one of the electroluminescent elements. A first power supply is connected to the thin film transistors and supplies a power supply voltage to the thin film transistors. The first power supply changes the power supply voltage in response to an output of the sensor. A second power supply is connected to the electroluminescent elements and supplies a reference voltage to the electroluminescent elements.
The invention also provides an organic electroluminescent display device that includes, for each of red, green and blue emissions, a plurality of electroluminescent elements for corresponding light emission. Each of the electroluminescent elements includes an anode, a cathode and a light emitting layer disposed between the anode and the cathode. The device also includes a plurality of thin film transistors and a sensor. Each of the transistors drives one of the electroluminescent elements. The device includes, for each of red, green and blue emissions, a power supply connected to the thin film transistors for supplying a power supply voltage. The power supply changes the power supply voltage in response to an output of the sensor. The device also includes a reference voltage supply connected to the electroluminescent elements for supplying a reference voltage.